In modern transportation of food a wide range of temperature levels must be maintained depending upon the particular food. It is important that the heating or cooling obtain an accurate desired temperature. When defrosting is needed it should be completed as rapidly as possible. To avoid error the manual adjustment that must be made by the operator should be as simple as possible, and it is desirable to have a single system that can perform a number of functions and the operation can be changed by the operator changing the setting of a single disc.